In a communication network traffic protection can be implemented in many different ways for various network topologies (e.g. chain, star, loop and mesh topology, while a mesh consists of at least two loops). Especially loop-based networks are attractive for network operators, because they offer the capability of quite easily and cost efficiently enabling redundancy in a network consisting of a relatively large number of nodes. In loop-based networks a redundant path for each node is provided by just one additional link that closes two adjacent branches to form a loop. For example, loop protection, i.e. provision of redundancy by way of a loop structure, is used today by many mobile network operators e.g. for 2G and 3G radio access networks (RAN), where network elements like e.g. base stations (BTS), NodeB's or stand alone transmission equipment represent the nodes in the loop.
In a radio access network typically point to point micro wave radio (MWR) links are used to interconnect the nodes. MWR links are sensitive against bad weather conditions (e.g. heavy rain), which may easily degrade the MWR link quality. In order to maintain the MWR link at an acceptable quality level, the modulation (and though the capacity) is reduced when the link quality degrades. This usually leads to a link break and requires the switching of the traffic (or part of the traffic) to an alternative route to avoid service degradation or to minimise an unfavourable effect to the service. Therefore redundancy is especially important in networks (e.g. radio access networks) where MWR links are used.
However, although micro wave radio links are taken herein as one example for radio links, the above also applies more or less to any kind of wireless link irrespective of the underlying technology.
Currently, protected traffic in a loop in a communication network, including for example an MWR-based RAN, is based on time division multiplexing (TDM) technologies. Examples thereof include synchronous digital hierarchy (SDH) and plesiochronous digital hierarchy (PDH). In this case, SDH or PDH frames are encapsulated and sent across a link. With TDM transport technologies, typically 50% of the available capacity is reserved for redundancy. Capacity allocations (e.g. including bandwidth on links) are fixed, which means there is no capacity or bandwidth flexibility. The capability of adaptive radio modulation to maintain data transfer in bad transmission conditions can not be exploited. Therefore, decreasing capacity on a link would lead to a complete loss of certain links. Such a behaviour is known as on/off characteristic.
The above-mentioned drawbacks in current loop-based (radio) networks could be obviated by the use of a packet-based transport technology such as for example Ethernet (IEEE 802.3). Due to its bandwidth flexibility, Ethernet is particularly suitable as a data link layer technology over an MWR link with variable capacity or bandwidth, like it is the case with adaptive modulation. When the link capacity or bandwidth of a MWR link changes, Ethernet connectivity is not lost, but is flexible to adapt to the new link conditions. That is, Ethernet transport does not exhibit an on/off characteristic. Therefore, although Ethernet is only one non-limiting example for a packet-based transport technology usable in this regard, the following mainly refers to Ethernet when some kind of such a packet-based and/or asynchronous transport technology is meant.
Packet-based transport e.g. using Ethernet will become especially important when high speed traffic is carried over Ethernet. As data traffic has a strong statistical nature, the capacity of a loop based network can be utilized more efficiently with a packed-based technology such as Ethernet.
However, there exists a problem that loops are not allowed in Ethernet-based transport networks, since Ethernet frames would circulate forever. So the loop has to be broken at some point, i.e. at some link between two nodes, so that Ethernet transport is enabled, thus e.g. facilitating MWR-based loop networks based on Ethernet.
Known solutions in the field of Ethernet-based loop and Ethernet loop protection like Resilient Packet Ring (RPR) and Ethernet Automatic Protection Switching (EAPS) do however not fit well with or are not giving the full benefit in hierarchical network architectures such as those of a radio access network, where there is no (or just a very limited amount of) traffic between the nodes in the loop and the main portion of traffic is upstream traffic.
In detail, Resilient Packet Ring (IEEE 802.17) is a complex layer-2 technology, which is independent of the underlying physical layer. The RPR concept is based on two counter-rotating rings designed to transport Ethernet frames efficiently e.g. in metro networks. There are no dedicated protection resources, and both rings transport traffic using shortest paths. RPR provides a fast protection switching (less than 50 ms). However, RPR's efficiency could be best utilized in networks and architectures, where the traffic is equally distributed between the nodes in the loop, but not in hierarchical (mobile) access networks.
Further, Ethernet Automatic Protection Switching (EAPS) is an exemplary solution for layer-2 loop protection, which is comparable to solutions such as Ethernet Protection Switched Rings (EPSR) and Ethernet Ring Protection (ERP). The solution has been documented in the informational Internet draft RFC3619. In this regard, it is to be noted that the terms loop and ring are to be understood as synonyms herein.
The EAPS ring consists of a master node and one or more transit nodes. The two ring ports of the master node are configured as primary port and secondary port. The master node blocks logically the secondary port except for a control VLAN (virtual local area network). The master node sends periodic health check packets from the primary port through the control VLAN towards the secondary port. When a fault occurs in the ring, the master detects this either by missing health check packets or by special fault detection packets generated by one of the transit nodes. In practice, the master node which is blocking the secondary port has to be located on the site where the traffic is forwarded upstream towards a controller. This is not optimal from load balancing point of view, especially in hierarchical architectures, such as for example hierarchical RAN architectures, because the optimal place for the break would be in the middle of the ring in respect of the master node. In addition, EAPS and similar solutions are on/off-type mechanisms without adaptation to available link capacities in the ring, therefore e.g. not allowing any load balancing. In a further known concept known as spanning tree, loops resulting from redundant paths are broken by use of the Spanning Tree Protocol (STP) algorithm. The STP breaks loops by disabling Ethernet switch ports so that the remaining active links build up a tree topology. In a failure case, when an active link breaks, STP calculates a new tree, taking then the appropriate so far disabled links into use. The original STP has meanwhile been superseded by the Rapid STP (RSTP), which converges faster. However, both STP and RSTP are on/off-type mechanisms without adaptation to available link capacities in the ring, therefore e.g. not allowing any load balancing.
Thus, the above solutions as such are mainly suitable for links with on/off characteristic, thus being not optimum for appropriately distributing the load across the working links of a loop.
Thus, a solution to the above problems and drawbacks is needed for providing a dynamic loop protection in communication networks.